The present invention relates to a mobile communications technology. More particularly, the present invention relates to a radio resource assignment technology for preventing interference at a sector boundary or at a cell boundary in a mobile wireless communications system.
As a mobile telecommunications technology advances, a ubiquitous network is expected to be implemented which frees users from restrictions, such as places where they use mobile terminal devices or speed at which they travel, and allows them to connect to a network more freely.
The mobile wireless communications system comprises a plurality of base station devices and a plurality of mobile terminal devices for performing wireless communications with the base station devices. The plurality of base station devices are disposed in dispersion, providing areas called cells, which enable wireless communications, within a range in which radio waves transmitted from each base station device are reachable. The base station device has sometimes a plurality of areas, referred to as sectors, which are provided by angularly dividing the cell through the use of a directional antenna and in which the radio waves are reachable. Typically, a sector is constituted of three domains by dividing the cell into three parts. The sector can also be regarded as a cell which is formed by angularly dividing a space using the directional antenna. It should be noted that the term cell is sometimes used below to mean to include both concepts of the sector and cell in the present invention.
The wireless communications system is structured to ensure that a communication path provided to a mobile terminal device is continuously handed over from one base station device to another as the mobile terminal device travels, and wireless communications is maintained while the mobile terminal device is travelling. Cell regions that are formed by the base station devices overlap one another to ensure that the handover between base station devices is performed without the communications being interrupted. When a mobile terminal device wirelessly communicates with a wireless base station in the region where cell regions overlap one another, the communications interferes with communications performed by another base station device which is located at a region where cell regions overlap one another. Such interference could produce waves disturbing the communications performed by another mobile terminal device, causing signal quality and throughput in the wireless communications to deteriorate.
A method of reducing such interference at the cell boundary or sector boundary is FFR (Fractional Frequency Reuse). This art aims to reduce interference at a particular frequency band to thereby improve throughput by causing a plurality of adjacent base station devices to share frequency resources in a coordinated manner, and weigh transmission power.
Other techniques for reducing interference include those described in Patent Document 1, Patent Document 2, and Patent Document 3. In Patent Document 1, a wireless base station extracts information on the amount of interference among cells from an uplink signal, generates uplink control information based on the information, transmits it to adjacent mobile communications terminal devices, and the terminal devices schedule uplink resources with reference to the uplink control information. In Patent Document 2, a transmission frame is divided in the time-base direction into a subchannel subset usage zone and a subchannel usage zone. The subchannel subset usage zone is divided in the frequency axis direction into a plurality of zones. A connection is assigned to a previously set zone in order of multiple priorities and defined as a priority zone, and a power value is assigned that is needed to improve the signal quality to a given level. In Patent Document 3, when user data is transmitted to a terminal station, a frame is used that comprises a segment region and a non segment region. A subchannel is assigned to a segment region. The subchannel differs for each sector and is used for the transmission of the user data to the terminal station existing in a sector edge and a cell edge which are divided in the time-base direction The non segment region is used for the transmission of the user data to the terminal station that does not exist in the sector edge and cell edge.
Turning now to standardization, a wireless communications scheme based on Orthogonal Frequency Division Multiplexing (OFDM), referred to as Long Term Evolution (LTE), is proposed by Third Generation Partnership Project (3GPP), which is one of standardization organizations. Communications between base station devices using LTE is described in Non-Patent Document 1.